A single G1 pony in a Bubble Mailer is approx. 4-5 ounces. And it is going to cost you roughly $12.75 to ship, if you can get it at the 4 ounce mark it then will be around $9.45. But be advised the prices I am quoting you are if you print your labels online.
I would suggest as Glacia mentioned to get it ready as if you were going to ship it and have your PO give you a quote. It is going to be the most accurate, especially if you do not have a scale to weigh it on.